1) Field of the Invention
The present invention relates to a thermal transfer image-receiving sheet. More particularly, the present invention relates to a thermal transfer image-receiving sheet capable of recording thereon thermally transferred dye or ink images or pictures in a clear and sharp form without a thermal curling thereof, at a high resolution and a high tone reproductivity, and capable of being smoothly moved through a thermal printer without fear of a blockage in the thermal printer, especially a dye-thermal transfer printer.
2) Description of the Related Arts
Currently there is enormous interest in the development of new types of color printers capable of recording clear full color images or pictures, for example, relatively compact thermal printing systems, especially sublimating dye thermal transfer printers.
The small sized dye thermal transfer full color printers are expected to be widely utilized as electronic camera printers and video printers.
In the dye thermal transfer printer, colored images or pictures are formed by superimposing a dye ink sheet composed of a substrate sheet and a dye ink layer formed on the substrate sheet and comprising a mixture of a sublimating dye with a binder on a dye image-receiving sheet composed of a dye image-receiving resinous layer formed on a substrate sheet in such a manner that the ink layer surface of the ink sheet is brought into direct contact with the dye image-receiving resinous layer of the dye image-receiving sheet, and the dye ink layer is partly heated by a thermal head of a printer in accordance with an input of electric signals corresponding to the images or pictures to be printed, to thermally transfer the dye images or pictures to the dye image-receivinq resinous layer.
It is known that a dye image-receiving sheet composed of a substrate sheet consisting of, for example, a biaxially oriented film comprising a mixture of a polyolefin resin with an inorganic pigment and a dye image-receiving layer comprising a dye-receiving polymeric material, for example, a polyester resin, polycarbonate resin or acrylic resin, is useful for recording thereon clear dye images, using the thermal printer as mentioned above. The above-mentioned film has a uniform thickness, a high flexibility and a low thermal conductivity, compared with that of a cellulosic pulp paper sheet, and therefore, is advantageous in that thermally transferred colored images thereon have an even color density and a strong color depth.
Nevertheless, when the dye image-receiving sheet having a substrate sheet consisting of a thermoplastic film or an oriented plastic sheet with microvoids is subjected to a thermal transfer printing operation, the stress created by a drawing operation in the film is released, and according, a shrinking of the film or sheet occurs. This shrinkage causes a curling or wrinkling of the dye image-receiving sheet, and thus a travel of the image-receiving sheet through the printer is disturbed. Also, the resultant curled prints exhibit a poor appearance.
To eliminate the disadvantages of the conventional image-receiving sheet due to the thermal properties of the substrate sheet, an attempt has been made to provide a substrate sheet comprising a core sheet consisting of a cellulosic pulp paper sheet which exhibits a very small thermal shrinkage, and coating layers adhered to the front and back surfaces of the core sheet and consisting of a monoaxially or biaxially oriented thermoplastic film. In this case, the relatively high roughness of the core paper sheet surface has an adverse influence on the surface property of the image-receiving resinous layer formed on the substrate sheet, and thus contacts of the image-receiving resinous layer surface with the ink sheet surface, and of the ink sheet surface with the thermal head, becomes uneven. This uneven contact lowers the quality of the resultant images on the image-receiving sheet, and further, lowers the reproducibility of the continuous tone color images.
Particularly, in a full color image dye thermal transfer printing system, there is a demand for an improvement of the image-receiving sheet by which the quality of the thermally transferred colored images is enhanced.
Also, there is a strong demand for an improvement of the close contacts of the thermal head to the ink sheet, and of the ink sheet to the image-receiving resinous layer, to enhance the accuracy of the thermal transfer of the dye images and to prevent the adverse influence imposed on the thermally transferred dye images due to the large amount of heat imparted by the thermal head.
Nevertheless, these demands have yet to be satisfactorily met.
Usually, the image-receiving resinous layer is formed by coating a coating liquid containing a dye-receiving resinous material dissolved in an organic solvent, on a surface of a substrate sheet and drying the coated coating liquid layer.
For example, Japanese Unexamined Patent Publication No. 61-297185 discloses a method of forming the image-receiving resinous layer from a resin solution by using a wire bar. This method is disadvantageous in that the resultant image-receiving resinous layer surface has a number of fine irregular streaks formed by the wire bar and the resultant rough surface of the image-receiving layer is a cause of an uneven color depth of the transferred images. To avoid the above-mentioned disadvantages, the printing operation must be carried out along the coating direction of the image-receiving resinous layer.
Also, to lower the stripe-shaped surface roughness of the image-receiving resinous layer, an attempt has been made to reduce the concentration of the resinous material in the coating liquid, to lower the viscosity of the coating liquid. This attempt is disadvantageous in that a large amount of heat energy becomes necessary for drying the coated coating liquid layer and a large amount of organic solvent must be used to dilute the coating liquid, and thus the cost of the production of the image-receiving sheet is increased.
Japanese Unexamined Patent Publication No. 62-211,195 discloses a method of forming an image-receiving resinous layer by coating an aqueous coating liquid containing a dye-receiving resinous material on a substrate sheet, coagulating the resultant coating liquid layer, and drying the coagulated resinous material layer while pressing the resinous material layer onto a surface of a cast drum to form a flat image-receiving resinous layer.
This method is disadvantageous in that the apparatus necessary for forming the image-receiving resinous layer is large and costly and only the aqueous coating liquid can be utilized, the resultant image-receiving resinous layer has a poor quality, and when the resultant image-receiving resinous layer is separated from the cast drum surface, a number of fine irregular marks are formed on the surface of the resinous layer.
In the conventional image-receiving sheet, various polyester resins are employed to form the image-receiving resinous layer. For example, to provide a polyester resin having a high dye thermal transfer rate and/or a large dye-receiving capacity, an attempt has been made to lower the glass transition temperature thereof. In this attempt, a dicarboxylic acid component comprising a mixture of terephthalic acid and other dicarboxylic acid and/or a diol component comprising a mixture of ethylene glycol and other diol compound, is used to provide a copolyester resin having a relatively low glass transition temperature.
Generally, it is considered that a lowering of the glass transition temperature can bring a corresponding lowering of the thermal transfer-starting temperature of the resultant resin, and thus an increase of the thermal transfer rate of the resin.
Nevertheless, a heat transfer rate and the heating temperature must be raised to increase the sensitivity of the dye-receiving resinous material. Also, the image-receiving layer formed from a resinous material having a low glass transition temperature exhibits a low mechanical strength at a high temperature, and therefore, the resultant image-receiving sheet cannot travel smoothly through the thermal transfer printer due to a fuse-adhesion of the image-receiving resinous layer. In view of these phenomena, the low glass transition temperature causes the resultant image-receiving resinous layer to exhibit a low thermal sensitivity, and accordingly, the concept of increasing the color depth of the thermally transferred dye images by lowering the glass transition temperature of the dye-receiving resinous material is not practical.
An image-receiving resinous layer having an enhanced sticking or fuse-adhesion resistance and a satisfactory storage stability can be obtained from a resinous material having a relatively high glass transition temperature, but this type of resinous material is disadvantageous in that the resultant image-receiving resinous layer exhibits an increased dye thermal transfer-starting temperature, and therefore, a lower image-transfer sensitivity than the resinous material having the relatively low glass transition temperature.
Japanese Unexamined Patent Publication No. 62-244696 discloses a dye-receiving resinous material consisting of a polyester resin containing a copolymerized aromatic polyol compound having a phenyl group.
Usually, the substrates of the image-receiving sheet and the ink sheet are both formed of a thermoplastic resin, and accordingly when the image receiving sheet is fed into and delivered from the printer, a static charge is created on the sheet and the smooth travel of the sheet through the printer is often obstructed by the static charge thereon.
To prevent the generation of the static charge, an antistatic agent is applied to the image-receiving sheet and/or the ink sheet, but when the image receiving sheet is supplied in the form of individual cut sheets to the printer, the antistatic treatment applied only to the ink sheet cannot prevent the occurrence of a static charge of the image receiving sheet. This static charge of the image receiving sheet also obstructs the smooth travel of the sheets through the printer, and undesirably enhances the adhesion of dust thereto.
To eliminate the above-mentioned disadvantages, an antistatic agent has been applied to one surface of the image-receiving sheet, but the antistatic agent layer formed on the image-receiving resinous layer has a low antistatic activity durability, and has an adverse influence on the dye-receiving capacity of the image-receiving resinous layer.
In another attempt to solve this problem, an antistatic agent was mixed with the dye-receiving resinous material in the image-receiving resinous layer. Note, in this case, the antistatic agent must have a satisfactory compatibility with the dye-receiving resinous material.
Generally, it is preferable that the dye-receiving resinous material is hydrophobic and the antistatic agent is hydrophilic, and thus it is very difficult to find an antistatic agent compatible with the dye-receiving resinous material. If the antistatic agent is completely dissolved in the dye-receiving resinous material, the resultant image-receiving resinous layer does not exhibit an antistatic property. Also, if the antistatic agent phase is completely separated from the dye-receiving resinous material phase, the resultant image-receiving resinous layer probably will not exhibit a satisfactory antistatic property.
Furthermore, even if the antistatic agent can exhibit an antistatic activity in the image-receiving resinous layer, this effect practically results in a poor dye-receiving capacity.
Under the above-mentioned circumstances, there is a strong demand for the provision of a new type of image-receiving sheet having a high surface smoothness and a satisfactory resistance to deformation, for example, curling or wrinkling, and capable of recording thermally transferred dye images thereon, with a high clarity, a uniform color density, and a high accuracy.